1. Field of the Invention
The present invention relates to a method of tracking control for an optical disc recorder and reproducer which discriminates a track deviation of an optical head and controls the track deviation in accordance with a result of discrimination when tracking an optical disc having a land/groove track structure.
2. Description of the Related Art
Recently, the demand for a large storage capacity has been increased in an optical write/read field or an optomagnetic write/read field. Accordingly, in an optical recording medium such as a digital versatile disc-random access memory (DVD-RAM), there has been proposed a so-called land/groove (L/G) write system for writing information both on a land track and a groove track of the DVD-RAM.
FIG. 1 is a block diagram of a conventional tracking control apparatus for a land/groove write type optical disc recorder and reproducer.
Referring to FIG. 1, the conventional tracking control apparatus includes an optical detector 11 for detecting a quantity of light of a laser signal reflected from an optical disc, a radio frequency (RF) amplifier 12 for amplifying a signal detected from the optical detector 11 as an RF signal, a first adder 13 for producing a tracking error signal TE by adding an offset value xcex4 to an output signal of the RF amplifier 12, a digital signal processing section 14 for switching a polarity of the tracking error signal TE and outputting a control signal in accordance with a switched signal during switchover of a land/groove of the tracking error signal TE, an actuator 15 for finely moving an optical pickup in accordance with the control signal outputted from the digital signal processing section 14, and a second adder 16 for adding an output of the actuator 15 and a disc eccentricity value xcex5, and feeding a position value x of an object lens of the optical pickup head back to a subtracter 17 to follow a desired track Xr.
The operation of the conventional tracking control apparatus for an optical disc recorder and reproducer as constructed above will be explained with reference to the accompanying drawings.
First, the eccentricity value xcex5 of the optical disc and the output of the actuator 12 are added by the second adder 16, and then feedback to the subtracter 17. This feedback signal x is compared with a desired tracking target value Xr. At this time, the light reflected from the optical disc is detected as an electric signal by the optical detector 11, and the detected electric signal is amplified as RF signal by the RF amplifier 12. This amplified RF signal is added to the offset value xcex4 to produce the tracking error TE signal.
Here, the offset xcex4 of the optical disc is produced due to the quantity of light among signals A, B, C, and D formed on the optical detector 11 of the optical pickup, an offset that may be produced from an IC (i.e., master or sleeve) being used, and a degraded track following of a sled servo for moving the optical pickup from an internal periphery to an external periphery of the optical disc. The tracking error signal TE is offset from a track center Tc as much as the offset value xcex4.
FIGS. 2a and 2b are waveform diagrams of the tracking error signal in the land/groove , tracks with respect to the offset xcex4. In case of xcex4=0 as shown in FIG. 2a, the tracking error signal TE follows the track center Tc with TEG(x) and TEL(x). In case of xcex4xe2x89xa00, As shown in FIG. 2b, the tracking error signal TE follows the track center Tc with TEG(x)+xcex4 and TEL(x)+xcex4, being apart from the track center Tc as much as the offset value xcex4.
The digital signal processing section 14, which receives the tracking error signal TE as described above, switches the sign (i.e., xe2x80x9c+xe2x80x9d or xe2x80x9cxe2x88x92xe2x80x9d) of the tracking error signal TE in accordance with the land/groove (L/G) signal of the track that the optical head 1 follows, and outputs the control signal u to the actuator 15 in accordance with a switched track error em.
Here, the digital processing section 14 is composed of a sign switching section 14a and a tracking controller 14b. The sign switching section 14a outputs the track error em by switching the sign of the tracking error signal TE by the L/G signal inputted according as the track that the optical head 1 follows is the land or the groove. The tracking controller 14b judges the deviation and the tilt of the optical disc by the switched track error em, and outputs the control signal u to the actuator 15.
The actuator 15 moves in a direction that the offset is removed according to the control signal u. At this time, the sled servo extracts a DC offset component of the tracking servo as a driving voltage of the actuator 15, and drives a sled motor to move the pickup. The second adder 16 feeds the position value x of the object lens of the optical head 1 back to the subtracter 17 to follow the desired track Xr by adding the output of the actuator 15 and the disc eccentric signal xcex5.
Meanwhile, as shown in FIGS. 2a and 2b, since the physical movement of the optical head in a radial direction r and the sign of the tracking error signal TE detected in a track direction t have a phase difference of 180xc2x0, respectively, in the land/groove, the sign switching section 14a switches the sign of the tracking error signal based on either of the land and groove according as the land/groove track is accessed. Also, the sign of the DC offset xcex4 is changed oppositely simultaneously with the switchover of the tracking error signal TE, and thus the tracking error signals having the different tracking deviation are inputted to the tracking controller 14b. 
Specifically, in the system, if the offset value added to the tracking error signal TE is xcex4 offset=0, the track error em in the radial direction r of the optical head 1 satisfies the following equation.
TEG=xe2x88x92TELxe2x80x83xe2x80x83[Equation 1]
Here, TEG is the tracking error signal on the groove track, and TEL is the tracking error signal on the land track.
Also, in the system, if the offset value is not 0 (i.e., xcex4 offsetxe2x89xa00), the track error em on the groove with respect to the movement of the optical head 1 in the radial direction satisfies the following equation.
em=TE=TEG+xcex4 offsetxe2x80x83xe2x80x83[Equation 2]
Meanwhile, in case of following the land of the track, the track error em is obtained by the following equation.
em=xe2x88x92TE=xe2x88x92(TEGL+xcex4 offset)=TEGxe2x88x92xcex4 offsetxe2x80x83xe2x80x83[Equation 3]
The tracking controller 14b receives the track errors obtained from the equations 2 and 3, and drives the actuator 15. At this time, since the offset xcex4 exists though the tracking controller 14b is ideally designed and the condition that a standby state error is zero (xe2x80x9c0xe2x80x9d ), the object lens is actually offset from and follows the track center in the standby state.
The land/groove switching state in a system where the offset is not 0 will be explained with reference to FIGS. 3a, 3b, and 3c. FIG. 3a is a view illustrating the variation of the tracking error signal TE when the object lens crosses in the radial direction during the land/groove switching operation, FIG. 3b is a view illustrating the time t and the movement of the object lens on the track, and FIG. 3c is a view illustrating the track error level for the time t.
In case that the offset (xcex4, {circle around (1)}) exists as shown in FIGS. 3a and 3b, the object lens in the standby state is actually offset ({circle around (6)}) from and follows the track center. If the offset value is not completely removed, the track error em having twice the offset (2xcex4, {circle around (2)}) is inputted to the tracking controller 14b during the switchover at the next land/groove switching time point.
Then, the tracking controller 14b outputs the control signal u to the actuator 15 to move the object lens in the directions ({circle around (2)},{circle around (4)}) that the track error is removed, and the actuator 15 moves the optical pickup. At this time, the object lens on the land/groove track moves in the directions ({circle around (7)}, {circle around (8)}) opposite to the track center as much as the offset xcex4 (FIG. 3b), and the shaking ({circle around (9)}) of the object lens of the optical pickup occurs due to the transition during the switchover on the land/groove tracks (FIG. 3c).
Specifically, if the offset exists in the tracking error signal TE inputted to the tracking controller 14b of the digital signal processing section 14, the track error em corresponding to 2 xcex4 abruptly occurs during the switchover of the land/groove tracks, and this causes the transition phenomenon ({circle around (9)}).
Meanwhile, if the tracking error signal TE, which crosses the track in the system where the offset exists (i.e., xcex4xe2x89xa00), is asymmetric, the track error em corresponding to 2xcex4 occurs during the land/groove switchover (FIGS. 4a and 4c), and thus the shaking of the object lens due to the transition occurs (FIG. 4b).
However, according to the conventional apparatus, since the track following position is changed (i.e., {circle around (1)}xe2x86x92{circle around (2)}) centering around the track center Tc when the track is switched from the land to the groove and vice versa, an instantaneous transition response occurs just after the switchover, and the shaking of the object lens ({circle around (7)}and {circle around (8)} in FIG. 3b) also occurs due to the transition phenomenon during the land/grove switchover.
Also, since the twice the offset value exists during the next land/groove switchover if the offset value is not completely removed, the track error corresponding to the offset value occurs. If this offset value is not removed, it exerts a bad effect on the tracking servo system.
Therefore, an object of the invention is to solve the problems involved in the related art, and to provide a method of tracking control for an optical disc recorder and reproducer which can prevent the track deviation by discriminating existence/nonexistence of the track deviation from the transition response that appears during the land/groove switchover, and optionally or automatically controlling the offset so that the transition response disappears.
In order to achieve the above object, there is provided a method of tracking control for an optical disc recorder and reproducer, comprising the steps of detecting a land/groove switching position of an optical disc; storing tracking error values for a predetermined time if the land/groove switching position is detected; and obtaining a mean value of the stored tracking error values for a predetermined number of times, judging whether the mean value is larger than a predetermined permissible tracking level, and if it is judged that the mean value is larger than the predetermined permissible tracking level, determining a direction of a track center, correcting the track center, and compensating for a DC offset.
Here, the mean value of the tracking error values is obtained by measuring the tracking error values at least once.
Also, the tracking error values are adaptively detected at the land/groove switching position even if a track servo is in a normal operation state to compensate for the DC offset.